1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an In-Plane Switching (IPS) mode LCD device and method for manufacturing the same to improve the light efficiency by changing the shape of an electrode.
2. Discussion of the Related Art
Demands for various display devices have increased as we develop into an informational society. Accordingly, much effort has been made to research and develop various flat display devices such as liquid crystal displays (LCDs), plasma display panels (PDPs), electroluminescent displays (ELDs), and vacuum fluorescent displays (VFDs). Some species of the flat display devices are already being used as displays of various equipments.
Among the various flat display devices, the liquid crystal display (LCD) device has been most widely used as a substitute for the cathode ray tube (CRT) because of thinness, lightness in weight, low power consumption and other advantageous characteristics. In addition to mobile-type LCD devices, such as displays for notebook computers, LCD devices have been developed to be used as computer monitors and, more recently, as televisions to receive and display broadcasting signals.
Despite continued technical developments in LCD technology applied to various fields, research in enhancing the picture quality of LCD devices has been lacking as compared to other physical features and advantages of LCD devices. The key to developing LCD devices to be used as general displays for various applications depends on whether high quality pictures, such as high resolution and high luminance, can be implemented on large-sized screens while still maintaining light weight, thin size, and low power consumption.
Generally, LCD devices include an LCD panel for displaying a picture image and a driving part for applying a driving signal to the LCD panel. The LCD panel includes first and second glass substrates bonded to each other at a predetermined interval, and a liquid crystal layer formed by injecting liquid crystal materials into the space between the first and second glass substrates.
On the first glass substrate (TFT array substrate), there are a plurality of gate lines arranged in a first direction at fixed intervals, a plurality of data lines arranged in a second direction at fixed intervals and perpendicular to the gate lines, a plurality of pixel electrodes in respective pixel regions defined by the gate lines and the data lines arranged in a matrix, and a plurality of thin film transistors (TFTs) switchable in response to signals on the gate lines to transmit signals on the data line to the pixel electrodes. The second glass substrate (color filter array substrate) has a light-shielding layer for shielding light from areas other than from the pixel regions, a color filter layer (R, G, B) for displaying colors, and a common electrode for generating a picture image.
The LCD device is driven according to optical anisotropic and polarizable characteristics of the liquid crystal material. Liquid crystal molecules are aligned using directional characteristics because liquid crystal molecules are long and thin. In this respect, an induced electric field is applied to the liquid crystal material to control the alignment direction of the liquid crystal molecules. By controlling the alignment direction of the liquid crystal molecules with the induced electric field, light is polarized and manipulated by the optical anisotropic properties of the liquid crystal, thereby generating a picture image. Currently, an active matrix type LCD, in which a TFT and a pixel electrode are connected and aligned in matrix form, is considered to be the best due to its high resolution and its ability to represent animated images.
Hereinafter, related art LCD devices will be described with reference to the accompanying drawings. FIG. 1 is an exploded perspective view of a Twisted Nematic (TN) mode LCD device according to the related art. As shown in FIG. 1, the TN mode LCD device according to the related art includes a lower substrate 1, an upper substrate 2, and a liquid crystal layer 3, wherein the liquid crystal layer 3 is formed between the lower substrate 1 and the upper substrate 2.
Specifically, the lower substrate 1 includes a plurality of gate lines 4, a plurality of data lines 5, a plurality of pixel electrodes 6, and a plurality of thin film transistors T. The plurality of gate lines 4 are formed on the lower substrate 1 in one direction at fixed intervals, and the plurality of data lines 5 are formed perpendicularly to the plurality of gate lines 4 at fixed intervals, thereby defining a plurality of pixel regions P. Plurality of pixel electrodes 6 are respectively formed in pixel regions P defined by the intersection of the plurality of gate and data lines 4 and 5, respectively. Plurality of thin film transistors T are respectively formed at intersecting portions of the plurality of gate and data lines 4 and 5. The upper substrate 2 includes a light-shielding layer 7 that excludes light from regions other than from the pixel regions P, R/G/B color filter layers 8 for displaying various colors, and a common electrode 9 for displaying a picture image.
Thin film transistor T includes a gate electrode, a gate insulating layer (not shown), an active layer, a source electrode, and a drain electrode. The gate electrode projects from gate line 4, and the gate insulating layer (not shown) is formed on an entire surface of the lower substrate 1. Then, the active layer is formed on the gate insulating layer above the gate electrode. The source electrode projects from the data line 5, and the drain electrode is formed on the opposite side of the source electrode. Also, the aforementioned pixel electrode 6 is formed of a transparent conductive metal having high transmittance, for example, ITO (Indium-Tin-Oxide).
In the aforementioned LCD device, liquid crystal molecules of the liquid crystal layer 3 on pixel electrode 6 are aligned by a signal from the thin film transistor T. Light transmittance is controlled according to the alignment of the liquid crystal molecules in the liquid crystal layer 3, thereby displaying a picture image. To align the liquid molecules, an LCD panel drives the liquid crystal molecules by applying an electric field perpendicular to the lower and upper substrates. This method achieves high transmittance and aperture ratio. Damage by static electricity to liquid crystal cells can be prevented since the common electrode 9 of the upper substrate 2 serves as the ground. However, a wide viewing angle is difficult to obtain using this technique.
To overcome this problem, an in-plane switching (IPS) mode LCD device has been recently proposed. Hereinafter, a related art IPS mode LCD device will be described with reference to the accompanying drawings. FIG. 2 is a plane view of a related art IPS mode LCD device. FIG. 3 is a cross sectional view along I-I′ of FIG. 2. As shown in FIG. 2 and FIG. 3, the related art IPS mode LCD device mainly includes a lower substrate 10, an upper substrate 20, and a liquid crystal layer 25, wherein the liquid crystal layer 25 is formed between the lower substrate 10 and the upper substrate 20.
The lower substrate 10 includes a gate line 11, a data line 12, a common electrode 13, and a pixel electrode 15. The gate line 11 and the data line 12 cross each other to define a unit pixel region. The common electrode 13 and the pixel electrode 15 are formed at a predetermined interval within the pixel region. Generally, the common electrode 13 is positioned between each pixel electrode 15 with some portions of the common electrode 13 overlapping with the pixel electrode 15 to form a storage capacitor.
Also, a thin film transistor TFT is formed on the lower substrate 10, wherein the thin film transistor TFT includes a gate electrode 11a; a semiconductor layer 26, and source and drain electrodes 12a and 12b; respectively. The gate electrode 11a projects from the gate line 11, and the semiconductor layer 26 is overlaps the gate electrode 11a. Gate insulating layer 14 is formed on the entire surface of the lower substrate 10 including the gate electrode 11a. The source and drain electrodes 12a and 12b are formed at both sides of the semiconductor layer 26, wherein the source electrode 12a is formed at a predetermined interval from the drain electrode 12b. In this state, the drain electrode 12b of the thin film transistor TFT is connected with the pixel electrode 15.
The common electrode 13 is formed at a predetermined interval from the pixel electrode 15, wherein the common electrode 13 is formed on the same layer as either the gate line 11 or the data line 12 when forming either respective line. In the drawings, the common electrode 13 is formed on the same layer as the gate line 11.
Insulating layer 16 is formed between the data line 12 and the pixel electrode 15, wherein the insulating layer 16 is formed of the same material as the gate insulating layer 14. For example, an inorganic insulating material such as SiNx and SiOx or an organic insulating material such as acryl, polyimide, BCB (BenzoCycloButene) and photo polymer may be used. Then, a passivation layer 17 and a first alignment layer 18 are sequentially formed on the entire surface of the lower substrate 10 including the insulating layer 16 and the pixel electrode 15.
The common electrode 13 is electrically connected with a common line 19, whereby the common electrode 13 receives a voltage signal. When a voltage signal is applied to the pixel electrode 15 through the drain electrode 12b; the common electrode 13 generates an IPS mode electric field, thereby driving the liquid crystal molecules of the liquid crystal layer 25.
On the upper substrate 20, there is a light-shielding layer 21 to prevent light leakage on the remaining portions of the lower substrate 10 except the pixel region. Upper substrate 20 further includes, a color filter layer 22 for obtaining colors red R, green G and blue B, an overcoat layer 23 for planarizing the color filter layer 22 having color films, and a second alignment layer 24 for defining the initial alignment of liquid crystal molecules. The first and second alignment layers 18 and 24 are rubbed at a pretilt angle of 2° to 5° such that the liquid crystal molecules are initially aligned in parallel to the lower and upper substrates 10 and 20.
The aforementioned drawings show an optical mode of a general IPS mode. In an initial state, light is not transmitted unit a voltage is applied (i.e. normally in a black state). On applying a voltage to the pixel electrode 15 and the common electrode 13, an electric field is generated between the two electrodes 13 and 15 formed on the same substrate. The liquid crystal molecules of the liquid crystal layer 25 are aligned along the electric field formed between the two electrodes 13 and 15. The internal light is then transmitted along the aligned liquid crystal molecules of the liquid crystal layer 25, thereby representing a white state.
During operation, aligning the liquid crystal molecules in a predetermined direction is difficult when applying the voltage to each electrode because the liquid crystal molecules corresponding to the common and pixel electrodes 13 and 15 are positioned in the area where the electric field is divided. Accordingly, in the display mode, disclination is generated at the portion where the electric field is divided. To prevent light leakage on the portions forming the common electrode 13 and the pixel electrode 15, the common electrode 13 and the pixel electrode 15 are formed using metal or an alloy of ITO and metal.
Both the common electrode 13 and the pixel electrode 15 are formed on the lower substrate 10. The liquid crystal layer 25 is formed between the lower and upper substrates 10 and 20 at a predetermined interval therebetween, and the liquid crystal layer 25 is driven by the electric field formed between the common electrode 13 and the pixel electrode 15 on the lower substrate. The liquid crystal layer 25 is formed of liquid crystal molecules having positive dielectric anisotropic characteristics, whereby the longitudinal axes of liquid crystal molecules are aligned along the direction of the electric field.
In the turn-off state, the IPS mode electric field is not applied to the common electrode 13 or the pixel electrode 15, and the alignment direction of liquid crystal molecules in the liquid crystal layer 25 is not changed. In the turn-on state, the IPS mode electric field is applied to the common electrode 13 or the pixel electrode 15, and the alignment direction of liquid crystal molecules in the liquid crystal layer 25 is changed, wherein the liquid crystal molecules are twisted at an angle of 45°.
FIG. 4 is a cross sectional view for explaining an operation of the related art IPS mode LCD device. Referring to FIG. 4, the common electrode 13 and the pixel electrode 15 are alternately positioned in the related art IPS mode LCD device.
In the related art IPS mode LCD device, the different voltages are respectively applied to the common electrode 13 and the pixel electrode 15, whereby the IPS mode electric field is generated between the two electrodes 13 and 15. Due to the liquid crystal molecules having the liquid crystal molecules having positive dielectric anisotropic characteristic, the liquid crystal molecules are aligned in parallel along the IPS mode electric field formed between the two electrodes. As shown in FIG. 4, a complete IPS mode electric field is formed in region A between the common electrode 13 and the pixel electrode 15. This field causes the liquid crystal molecules to become aligned in parallel to the field. However, in region B above the common electrode 13 and the pixel electrode 15, only a partial IPS mode electric field is formed. Consequently, the liquid crystal molecules in Region B do not become completely aligned in parallel.
In the IPS mode LCD device, the common electrode 13 and the pixel electrode 15, positioned in the pixel region, are formed of the light-shielding metal material. The light shielding material blocks about 15% of light emitted from a backlight unit.
As compared with a non-IPS mode LCD device, the IPS mode LCD device has lower light efficiency. In order to overcome this problem, the backlight unit uses more power. However, high power consumption by the backlight unit is disadvantageous for small-sized mobile products, such as mobile phones, notebook computers, PDAs and the like. Accordingly, the related art IPS mode LCD device has the following disadvantages.
First, the common electrode and the pixel electrode have rectangular cross sections. Accordingly, even though a complete IPS mode electric field is formed between the common electrode and the pixel electrode, the rectangular cross sections prevent IPS mode electric field from being formed above the common electrode and the pixel electrode. This non-IPS region formed above the common electrode and the pixel electrode prevents the liquid crystal molecules from becoming completely aligned above the common electrode and the pixel electrode.
Secondly, the common electrode and the pixel electrode of the related art IPS mode LCD device are formed of the light-shielding metal material in the pixel region. This configuration blocks about 15% of the light emitted from the backlight unit. Accordingly, even though the IPS mode LCD device has a wider viewing angle in compared to the non-IPS mode LCD device, the IPS mode LCD device has lower light efficiency. In order to overcome this problem, the backlight unit for the IPS mode LCD device uses more power. High power consumption of the backlight unit is disadvantageous for small-sized mobile products, such as, mobile phones, notebook computers, PDAs and the like. Without the appropriate amount of power provided to these products, proper luminance is difficult to obtain in the mobile products. Accordingly, even though the IPS mode LCD device has a wide viewing angle, its low light efficiency hinders the competitiveness of the IPS mode LCD device over non-IPS mode LCD devices.